1. Field of the Invention
The present invention relates to an photoelectric conversion apparatus for receiving an optical signal from a subject, and an image pickup device using the photoelectric conversion apparatus.
2. Related Background Art
Some photoelectric conversion apparatus output a signal having an amplitude proportional linearly to the quantity of input light, and other photoelectric conversion apparatus output a signal having an amplitude obtained by converting the quantity of input light logarithmically. One type of the latter logarithmic compression photoelectric conversion apparatus utilizes a logarithmic amplifier (hereinafter referred to as a LOG amp).
FIG. 10 shows a conventional logarithmic compression photoelectric conversion apparatus using a LOG amp. In the figure, a reference numeral 501 designates a photodiode; reference numerals 502 and 503 designate an operational amplifier (hereinafter referred to as an OP amp) severally; reference numerals 504 and 505 designate a diode severally; reference numerals 506 and 507 designate a constant voltage input terminal severally; a reference numeral 508 designates a constant current source; and a reference numeral 509 designates an output terminal.
The voltage at the cathode terminal of the photodiode 501 is the voltage at the constant voltage input terminal (the reference input terminal) 507 of the OP amp 502 owing to the imaginary short of the OP amp 502. The voltage at the constant voltage input terminal 507 is designated by a reference character Vc here. If the voltage at the anode terminal of the photodiode 501 is equal to the voltage Vc or less, the photodiode 501 is reversely biased.
When light enters the photodiode 501, a photoelectric current Ip proportional to the incident light flows through the photodiode 501. The photoelectric current Ip flows from the output terminal of the OP amp 502 to the constant voltage input terminal 506 through the diode 504 and the photodiode 501 in order.
In this case, supposing that the voltage at the constant voltage input terminal 507 is Vc and the voltage of the output terminal of the OP amp 502 is V1, the voltage V1 can be expressed:V1=(qT/k)×ln(Ip/Is)+Vc  Expression 1where Is designates the reverse direction saturation current of the diode 504.
That is, the OP amp 502 outputs the output proportional to the logarithm of the quantity of the entered light (the photoelectric current Ip). Consequently, an input/output characteristic having a wide dynamic range can be obtained.
Moreover, the circuit comprising the diode 505, the OP amp 503, the constant current source 508 and the output terminal 509 is a circuit for compensating the dispersion of the reverse direction saturation current Is of the diode 504. Supposing that the voltage of the output terminal 509 is designated by a reference character Vout and the current flowing to the constant current source 508 is designated by a reference character Iref, the following expression can be obtained.Vout=−(qT/k)×ln(Iref/Is)+V1
By putting the expression 1 in the place of V1, and by supposing that the characteristics of the two diodes 504 and 505 are the same, the following expression can be obtained.Vout=(qT/k)×ln(Ip/Is)−(qT/k)×ln(Iref/Is)=(qT/k)×ln(Ip/Iref)  Expression 2Consequently, the output voltage Vout, which does not depend on the reverse direction saturation current of the diode 504, can be obtained.
As the diode 504 in this example, a bipolar transistor 510 connected in a diode connection is generally used as shown in FIG. 11.
FIG. 9 is a sectional view showing the structure of a cross section of a conventional bipolar transistor to be used in a photoelectric conversion apparatus. In the figure, a reference numeral 61 designates a p-type semiconductor substrate; a reference numeral 62 designates an n-type epitaxial layer used as a collector region; a reference numeral 63 is a p-type base diffusion layer; a reference numeral 64 designates an n-type emitter diffusion layer; a reference numeral 66 designates an n-type diffusion layer for taking out the collector region; a reference numeral 65 designates p-type diffusion layer; and a reference numeral 68 designates an n-type embedded diffusion layer. These components constitute an NPN transistor having the collector region of the n-type epitaxial layer 62, the base region of the p-type base diffusion layer 63, and the emitter region of the n-type emitter diffusion layer 64. By adopting the structure described above, it becomes possible to separate electrically the bipolar transistor from the semiconductor substrate 61. Incidentally, if such a bipolar transistor is used as a p-n junction diode simply, the bipolar transistor can be used equivalently as p-n junction diode by connecting the collector and the base of the transistor in common.
However, because the bipolar transistor is used as the p-n junction diode by connecting the collector and the base of the transistor in common, each terminal of the emitter, the collector and the base of the bipolar transistor is used at intermediate voltages, which are neither power supply voltages nor the ground voltage.
Accordingly, it is necessary to adopt a device structure in which each terminal is separated from a substrate. The structure makes the manufacturing process of such a transistor having the structure be complicated, and the costs of the manufacturing increase.